What are residual stresses? 
 
Residual stresses are those stresses which remain in a component following manufacture, processing, fabrication or assembly.
 
How can you classify the types of residual stress?
 
Residual stresses may be classified by the scale over which they are significant, and hence the type of measurement technique used to study them. Macroscopic residual stresses exist over large distances whilst microscopic residual stresses operate over the grain scale or the atomic scale of the material.

Residual stresses are sometimes classified according to their origin (e.g. thermal or elastic mismatch).
 
How do residual stresses arise in a material?
 
Residual stresses originate from a variety of sources. Macroscopic stresses can arise from heat treatment, machining, secondary processing and assembly. Microstructural stresses often result from the CTE (coefficient of thermal expansion) mismatch between phases and constituents or from phase transformations.

Both types may be present at any one time in a material or component. 

How do residual stresses affect performance ?
 
The importance of residual stresses depends on the particular material, component and application. No single answer can be given. Residual stresses can be beneficial or detrimental to performance; they may be critical or insignificant. Each case must be examined on it's own merits.

What materials contain residual stresses?
 
It is unlikely that any component will be entirely free from residual stresses induced during manufacturing and processing. Residual stresses may be present in engineered components, thin films and coatings, composites and multiphase materials. Depending on the manufacturing process, the type of material and the component criticalility, the levels of residual stress may or may not be significant.
 
How do we measure these residual stresses?
 
There are a variety of techniques available. NPL has a direct involvement in measurement technologies, and our work on residual stress measurement discusses these in more detail.

Mechanical methods such as hole drilling, curvature measurements, and crack compliance may be used to measure residual stress by changes in component distortion.
Diffraction techniques such as Electron, X-Ray and neutron diffraction are also available.
A number or other methods including magnetic, ultrasonics, piezospectroscopy photoelastic methods and thermoelastic techniques are also being developed.